1. Field of the Invention
The present invention relates to an ink jet printer and an ink jet type for printing by ink discharged from a nozzle by expanding and shrinking a pressure generation chamber filled with the ink by an electro-mechanic converter consisting of a piezoelectric element or the like. In particular, the present invention relates to an ink jet printer and an ink jet printing method in which the pressure chamber is expanded in a stepped manner so as to discharge fine ink droplets at a high speed, thus improving the image quality, and the pressure chamber is contracted prior to ink discharge so that a wide range of ink droplet size is discharged, realizing a highly accurate gradation printing. The present invention also improve the service life of the electro-mechanic converter such as a piezoelectric element.
2. Description of the Related Art
Conventionally, the ink jet printer has been used as a printing apparatus for recording on a recording paper a data from an electronic processing apparatus such as a computer.
The ink jet printer uses a piezoelectric element or other electro-mechanic converter to expand and contract a pressure generation chamber filled with ink so as to discharge the ink from a nozzle communicating with the pressure generation chamber for printing on a recording paper. Since the ink jet printer can be manufactured with a simple configuration of a small size at reasonable costs, it is widely used for business and private use at home.
In general, a printer is required to have a high quality and high speed printing capability. Especially with the recent spread of personal computers, a high speed and high density recording is strongly desired.
For performing a high-density recording in the ink jet printer, it is necessary to reduce the ink droplet size, for example, by reducing the diameter of the nozzle.
However, a nozzle of a small diameter has difficulty in production as well as has a problem of clogging, deteriorating the reliability. Accordingly, reduction in the nozzle diameter has a limitation.
To cope with this, it has been considered to reduce the ink droplet size by controlling the expansion and contraction speed of the pressure generation chamber.
As is known, in the ink jet printer using a piezoelectric element as an actuator of the pressure generation chamber, a drive voltage signal is applied to deform the piezoelectric element. The drive voltage signal which has been used is a trapezoidal wave as shown in FIG. 14.
As shown in this figure, in a conventional ink jet printer, a drive voltage V1 is increased for time T1 and applied for time T2. This deforms the piezoelectric element to push the wall of the pressure generation chamber.
When the wall is pushed, the pressure generation chamber is contracted to discharge the ink filled inside.
When the drive current is decreased to V0, the piezoelectric element returns to its previous form and the pressure generation chamber returns to its previous volume, so that ink is filled from a common ink tank communicating with the pressure generation chamber.
This contraction and expansion of the pressure generation chamber are repeated to discharge ink for printing a predetermined image or character on a printing paper.
Accordingly, in order to obtain a smaller ink droplet in the ink jet printer using such a trapezoidal drive waveform signal, what can be done is to reduce the drive voltage V1 and the voltage application time T1.
However, in order to set the voltage application time T1 to a smaller value, there arise various problems such as current limit in a circuit, responsibility of the piezoelectric element, resonance, and the like.
Moreover, if the drive voltage V1 is set to a too small value, there arises a non-discharge region where the ink column is not broken.
For this, if only the voltage application time T1 and the drive voltage V1 are simply reduced, the ink droplet diameter and the ink droplet speed are in a region "a" shown in FIG. 15. It is difficult to obtain a droplet smaller than the nozzle diameter.
It should be noted that FIG. 15 show as regions "a" to "f" as the maximum values of the ink droplet speed which can be obtained constantly when driven by various drive methods in the ink jet printer including the conventional technique and present invention which will be detailed later.
Thus, in the ink jet printer, it is difficult to obtain a very small droplet by reducing the drive voltage signal of the conventional trapezoidal waveform and reducing the voltage application time.
To cope with this, there has been suggested a method for modifying the piezoelectric element drive waveform signal to other than trapezoidal for discharging an ink droplet.
For example, Japanese Patent Publication A55-17589 [1] discloses a Pull-Push Method (so-called "hikiuchi" in Japanese), i.e., starting an ink discharge at the moment when the ink meniscus is pulled into the nozzle.
FIG. 16 shows a waveform of the drive signal of this pull-push drive method. Prior to contract the pressure generation chamber to discharge an ink droplet, the pressure generation chamber is once expanded. This brings about two merits.
Firstly, in this pull-push drive, the ink meniscus is pulled into the nozzle when discharge is started. Accordingly, the ink column being discharged is formed more slender than the case of the conventional trapezoidal waveform shown in FIG. 14. This enables to make smaller the ink droplet discharged.
Moreover, in this drive method, even if the voltage is lowered before ink droplet discharge completion so as to take back the ink (expansion of the pressure generation chamber), the ink droplet is broken off from the ink meniscus. Thus, it was possible to reduce the ink droplet size.
Accordingly, when using the drive signal of FIG. 16, it is possible to obtain stable ink discharge even if the voltage application time T3 maintaining the contracted state of the pressure generation chamber at a smaller value than T2.
Thus, by using the drive signal of FIG. 16, it is possible to obtain a smaller ink droplet than when using the trapezoidal wave of FIG. 14 for drive.
On the other hand, Japanese Patent Publication A59-133067 [2] suggests a drive method for applying a voltage signal so as to pull back the ink into the nozzle before the ink droplet discharge is complete.
As shown in FIG. 17, in this drive method, the drive signal is triangular, where the voltage application maintaining time T2 in the conventional drive signal is made zero, so that the ink column during ink discharge is broken earlier.
This makes it possible to obtain a smaller size of ink droplet than when using the trapezoidal wave of FIG. 14 for drive.
Furthermore, Japanese Patent Publication B4-36071 [3] suggests a drive method of rapid meniscus pulling and maintaining the state to discharge a very small droplet.
As shown in FIG. 18, in this drive method, the drive signal has a reversed trapezoidal waveform, wherein the contraction time T4 of the pressure generation chamber is reduced and the bias voltage V2 is increased, so that a protrusion is formed at the center of the meniscus during a contraction maintaining time T5 of the pressure generation chamber.
This protrusion is broken off from the meniscus and becomes an ink droplet. Thus, it is possible to obtain an ink droplet having a diameter smaller than the nozzle diameter.
However, the aforementioned ink jet printers have a problem that it is impossible to-obtain both of the ink droplet size reduction and an appropriate ink droplet speed.
Firstly, in FIG. 16, by using the method disclosed in Citation [1], it was possible to obtain an ink droplet size and ink droplet speed shown by region "b" in FIG. 15. In comparison to the region "a" of the drive method of FIG. 14, it was possible to obtain a smaller ink droplet size at the same speed.
This is because the ink column formed is thinner than in the drive method of FIG. 14 and the force pulling the ink droplet toward the nozzle after discharge becomes smaller.
However, in the drive method of FIG. 16, in order to further reduce the ink droplet size, it is necessary to reduce the voltage V3 to be applied. There arises the same problem as the case of the trapezoidal wave signal shown in FIG. 14, and it is difficult to discharge an ink droplet smaller than the nozzle diameter.
Moreover, in the drive method of Citation [2], it was possible to obtain the ink droplet size and ink droplet speed indicated by region "c" in FIG. 15.
In comparison to the region "a" of the drive method of FIG. 14, it is possible to discharge a smaller ink droplet.
However, in this method, the ink column at the initial stage of the ink discharge is formed by the same phenomenon of the drive method of FIG. 14. Accordingly, like in the case of simple trapezoidal wave, it is impossible to make the head of the ink column smaller than the nozzle diameter. Thus, it is impossible to discharge an ink droplet smaller than the nozzle diameter.
Furthermore, in the drive method of Citation [3], it was possible to obtain the ink droplet size and ink droplet speed shown by region "d" in FIG. 15.
In comparison to the region "a" of the drive method of FIG. 14, it is possible to discharge an ink droplet of sufficiently small size.
However, in this method, it is impossible to obtain a sufficient ink droplet speed because the energy used for the ink droplet discharge is only the ink meniscus restoration force and the ink inertia flow.
That is, in this drive method, it is possible to obtain an almost sufficient result for the ink droplet size reduction, but it is impossible to obtain a sufficient ink droplet discharge speed. When the ink droplet discharge speed is not sufficient, the shooting range of the droplet may be shifted. As a result, it is difficult to perform a clear printing.
Thus, the aforementioned drive methods of the ink jet printer can reduce the ink droplet size but this is accompanied by reduction in the ink droplet speed. As a result, it has been difficult to realize clear printing at a high speed with a very small ink droplet.